Every protein has its own favorite conditions, but I've had success
storing many soluble proteins in buffer containing 10% glycerol at
-80 degrees, keeping protein conc. greater that 1 mg/ml if possible.
As for short term storage, I've found a difference between storing
proteins on wet ice (i.e. 0 degrees) and storing in the refrigerator
or cold room. So you might want to try keeping them on wet ice,
replenishing the ice when necessary, when you don't want to store
them for a long time. Also, if oxidation of thiols is a problem
with your protein, inclusion of DTT and covering the solution with a
blanket of argon may preserve its activity.
John <newitt at ncifcrf.gov>